1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly, to a metering method capable of accurately metering a resin and a control apparatus for the injection molding machine.
2. Description of the Related Art
In an in-line screw-type injection molding machine, a screw in a heating cylinder is rotated to melt and knead a resin material, and the molten resin is supplied under pressure to the distal end portion of the heating cylinder. Pressure control is carried out as the screw retreats. When the screw reaches a preset metering point, its rotation and retreat are stopped, and the resin is metered. In a pre-plasticization-type injection molding machine, a screw is rotated for pre-plasticization, and a molten resin is supplied to the distal end portion of the cylinder. A plunger retreats under the pressure of the resin that is supplied to the distal end portion of the cylinder. When the plunger reaches a preset position, the molten resin is metered. After this metering operation, the screw or plunger is advanced so that the molten resin is injected into a mold to fill it.
In order to enhance the quality of molded parts, variation of the fill of the molten resin in the mold must be reduced.
Injection is carried out after the screw or plunger retreats to the position of the metering point. If the metered molten resin is directly injected into the mold to fill it as this is done, an accurate fill can be obtained, so that the quality of the molded parts can be kept uniform without any variation in the weight of the molded parts. In this case, however, the metered resin quantity changes as an injection process is started after the metering operation is finished, so that an accurate molten resin quantity cannot be injected. Various techniques have been proposed to solve this problem.
In the metering process, the molten resin that is produced as the screw rotates is supplied to the distal end portion of the cylinder. A valve, such as a check valve or check ring, is provided on the distal end of the screw, whereby the molten resin at the distal end portion of the screw can be injected into the mold without flowing backward in the injection process. According to a proposed technique, the molten resin moves between the distal and rear end portions of the screw through the valve during the time interval between the end of the metering operation and the start of the injection. Thus, the metered resin quantity can be prevented from changing, so that the accurately metered resin can be injected.
(a) In the pre-plasticization type injection molding machine, the plunger retreats so that the pressure of the molten resin at the distal end portion of the screw is zero after the metering operation is finished. Thereafter, the plunger is advanced, and the check valve is causes to retreat to close a resin passage from the rear end portion of the cylinder to its distal end portion. Thus, the molten resin can be prevented from flowing backward, so that variation of the metered resin quantity can be eliminated (e.g., Japanese Patent Application Laid-open No. 2-147312).
(b) If the screw retreats to the metering point, in the in-line injection molding machine, its rotation and retreat are stopped. Thereafter, the screw is rotated reversely to lower the resin pressure at the rear end portion of the cylinder so that the check valve retreats to close the resin passage. Injection is started after the resin is sucked back. By doing this, variation of the metered resin quantity can be eliminated (e.g., Japanese Patent Application Laid-open No. 11-240052).
(c) Likewise, the screw is rotated reversely after the metering operation is finished, or the resin is sucked back as the screw is rotated reversely, and the resin passage is closed by means of the check ring. Injection is started thereafter (e.g., Japanese Patent Application Laid-open No. 10-16016).
(d) After the metering operation is finished, moreover, the resin is sucked back, the screw is then rotated reversely, and the resin passage is closed by means of the ring valve. Injection is carried out thereafter (e.g., Japanese Patent Application Laid-open No. 9-29794).
According to the conventional technique described above, fluctuation of the metered resin quantity that is attributable to, for example, a backflow of the molten resin at the distal end portion of the cylinder during the injection can be prevented.
The following is a description of a control technique that ensures an accurate metered resin quantity.
(a) A plurality of sets of back pressure commands and screw rotational frequency commands are prepared, and the combined back pressures and screw rotational frequencies are controlled in association with one another. When the metering process is finished, the screw rotational frequency and the screw retreating speed controlled and adjusted to zero, whereby the metering is made uniform (e.g., Japanese Examined Patent Publication No. 1-26857).
(b) Further, a stable metered resin quantity can be obtained by gradually lowering the screw rotating speed used before the metering operation is stopped with respect to a retreat position of the screw (e.g., Japanese Examined Patent Publication No. 64-6931).
The screw, a motor that rotates the screw, and a transmission mechanism that transmits motor torque to the screw have inertia. If the motor for screw rotation is stopped to interrupt the screw rotation when the screw reaches the position of the metering point, therefore, the screw rotation cannot be stopped in a moment under the influence of the inertia. Thus, the inertia causes the screw and the screw rotating mechanism to overrun. Thereupon, the resin quantity changes correspondingly, so that the metered resin quantity fails to be equal to a preset value. Since this metered resin quantity that causes the overrun is not controlled, it is only apparently stable, and fluctuates actually. In consequence, the actual weight of molded parts varies.
If a high screw rotational frequency is used with the same preset metering completion position, it takes much time to stop the screw, so that the metered resin quantity involves a surplus quantity that is attributable to the overrun. Therefore, the metered resin quantity is liable to increase and may vary depending on the molding conditions. Thus, settling the molding conditions requires an extra hard operation, such as fine adjustment of the preset metering completion position. Further, the aforesaid method in which the rotational frequency of the screw is gradually lowered as the metering completion position is approached requires time. In consequence, the metering process takes too much time.